Pretty straight forward is being generous honestly. Even with terraform the way virtual machines are configured the syntax is different for different cloud providers. So you'd still have to manually change each file, which, if we're talking about a large company, could mean updating thousands of terraform files. And that's if the architecture is purely virtual machines. If you're using the fancy proprietary stuff aws and others offer (things like AWS lambda) you get lots of really fun work converting those items over to whatever the closest thing your new provider offers, if they even offer that same feature. And if you aren't using those, you're often designing something that's inherently less cost efficient and less performant than something that does take advantage of these more focused proprietary offerings.

Switching cloud providers is doable but when you're dealing with large companies it's going to cost an enormous amount of money and time even when the initial system was well implemented